Positive-displacement compressors are commonly used in gas and refrigeration service, liquid, such as oil or water, is often injected into these compressors in order to lubricate the mechanism. The liquid injection also serves to seal machine clearances to prevent leakage of the compressed fluid and further serves to absorb the heat of the compression cycle itself.
The injected liquid is separated from the compressed fluid in a discharge receiver resulting in compressed fluid which is free of injected liquid and separated liquid. The separated liquid is cooled and reinjected into the compressor to complete the cycle. Flow of the rejected liquid back into the compressor is maintained by the compressor differential pressure and is controlled by an orifice or other pressure reducing device.
Some of the injected liquid will leak from the compression pockets of the compressor through the mechanism clearances towards the compressor inlet where it is drawn back into the compressor in an uncontrolled manner with the inlet gas or vapor. Liquid that is recirculated with the compressor in this way may be several times the amount of liquid injected into the compressor.